DC-AC power converters are part of the general power electronic converters family and are designed and operated to convert electrical energy from one stage voltage, current and/or frequency to another. Historically, DC-AC converters are referred to as inverters, and that term is used throughout this document. Inverters, as with other power electronics converters, are composed of groups of switching elements and are operated in a particular sequential manner to produce outputs with predefined specifications (voltage, current, and/or frequency). In general, power electronics converters operate by switching their elements in either full ON or full OFF modes in a sequential periodic manner to meet sets of predefined conditions on the output stage, as well as compliance with fundamental conditions for switching circuits. These conditions are required to avoid creating short circuits across the DC supply and to provide each switching element with the required time for changing its status from ON to OFF or OFF to ON. Adherence to these conditions by certain sequential switching methods produces AC outputs. However, outputs of these inverters contain different frequency components in addition to the desired fundamental frequency component. Such frequency components can create undesired features in the AC outputs as well as various levels of operational imperfections or inefficiencies.
The use of inverters is wide spread, and there are a variety of modulation techniques for switching the circuit elements of inverters to control both single phase and polyphase outputs. Generally, the two main types of inverters are single-phase (1φ) inverters and three-phase (3φ) inverters, and the literature is replete with topologies of 1φ and 3φ designed for particular tasks. Among the existing inverter modulation techniques are the pulse width modulation (PWM) and its different and improved versions, including selected harmonic elimination (SHE), random pulse-width modulation (RPWM), hysteresis-band current control (HBCC), delta modulation (DM), and other techniques. An inherent inefficiency of each of these methods is the reliance on a carrier frequency (or a band of carrier frequencies) to deliver the switching signals to the inverter. Spectral analysis of inverter outputs switched using these methods identify power deviation from the desired output frequency to these carrier harmonic frequencies (or frequency bands).
Inverter systems are being extensively utilized in various industrial applications including variable-speed motor drives (VSD), power quality improvement, renewable energy utilization, etc. An important characteristic of an inverter system for such applications is the ability to transfer high power from the DC side (input side) to the AC side (the output side) over a relatively wide range of output frequencies, in a manner which maximizes the amount of the energy on the AC output of the inverter in the chosen fundamental frequency components.
There are two traditional approaches for maximizing the energy concentration in an inverter's output fundamental frequency components. The first approach is based on minimizing the energy allocated in the undesirable harmonic components by calculating the switching times before the inverter is operated. Since this approach involves solving non-linear equations, it demands high level of computational and storage capabilities. One of the disadvantages of this approach is the complexities associated with controlling the inverter output. The second approach is based on generating switching signals with randomized frequency. One of the disadvantages of this approach is the reduction of the inverter overall efficiency due to the increase in the switching losses.
The prior art contains many examples of voltage source modulated power inverters capable of producing various waveforms. The modulation techniques in the prior art are mostly developed based on the load requirements, switching circuit capabilities, availability of the hardware to accommodate the implementation of the desired technique, etc.
There is a need for an inverter modulation technique that is developed and tested in correlation with the inverter function itself. There is a need for an inverter modulation technique that alleviates the reliance on a carrier signal (or band of carrier signals) to implementing switching. There is a need for an inverter modulation technique with improved response characteristics over a variety of loads.